CARDNAME<\/td> the ALSA card name of the sound interface, when it differs from Device<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nPreparing the Operating System<\/h2>\n\n\n\n The system starts from a fresh install with network access and a user with sudo rights. The package index is refreshed and the installed packages are brought up to date first.<\/p>\n\n\n\n
sudo apt update\nsudo apt upgrade -y<\/code><\/pre>\n\n\n\nWinlink sessions and APRS-IS logins depend on an accurate clock. Ubuntu enables time synchronization by default, and the state is confirmed with:<\/p>\n\n\n\n
timedatectl<\/code><\/pre>\n\n\n\n Local time: Wed 2026-11-04 13:40:12 UTC\n Universal time: Wed 2026-11-04 13:40:12 UTC\n RTC time: Wed 2026-11-04 13:40:12\n Time zone: Etc\/UTC (UTC, +0000)\nSystem clock synchronized: yes\n NTP service: active\n RTC in local TZ: no<\/code><\/pre>\n\n\n\nThe packages for every part of the build install in one command. The list covers the compiler and audio libraries needed to build XARA, Dire Wolf itself, the ALSA tools, and the helpers used later.<\/p>\n\n\n\n
sudo apt install -y git build-essential pkg-config rustc cargo \\\n libasound2-dev libpulse-dev alsa-utils direwolf usbutils \\\n unzip curl ufw python3<\/code><\/pre>\n\n\n\nThe Ubuntu 26.04 Rust toolchain (1.93) satisfies XARA’s minimum version of 1.85.<\/p>\n\n\n\n
Identifying the USB Interface<\/h2>\n\n\n\n With the interface connected, the operating system lists it as a USB device and as a sound card.<\/p>\n\n\n\n
lsusb | grep -i c-media<\/code><\/pre>\n\n\n\nBus 002 Device 002: ID 0d8c:0012 C-Media Electronics, Inc. USB Audio Device<\/code><\/pre>\n\n\n\nThe vendor ID 0d8c and product ID 0012 identify this interface. Other CM108\/CM119 interfaces report different product IDs, and the udev rule later in this article uses whichever pair lsusb<\/code> shows.<\/p>\n\n\n\ncat \/proc\/asound\/cards<\/code><\/pre>\n\n\n\n 0 [Device ]: USB-Audio - USB Audio Device\n C-Media Electronics Inc. USB Audio Device at usb-0000:02:1b.0-1, full speed<\/code><\/pre>\n\n\n\nThe card is named Device<\/code> in square brackets. That name appears in the ALSA configuration, so it stays valid even if the card number changes.<\/p>\n\n\n\narecord -l\naplay -l<\/code><\/pre>\n\n\n\n**** List of CAPTURE Hardware Devices ****\ncard 0: Device [USB Audio Device], device 0: USB Audio [USB Audio]\n Subdevices: 1\/1\n Subdevice #0: subdevice #0\n**** List of PLAYBACK Hardware Devices ****\ncard 0: Device [USB Audio Device], device 0: USB Audio [USB Audio]\n Subdevices: 1\/1\n Subdevice #0: subdevice #0<\/code><\/pre>\n\n\n\nThe interface also exposes a HID node. That node carries the GPIO that keys the radio. In a virtual machine the emulated USB tablet creates a HID node as well, so the vendor and product IDs identify the right one.<\/p>\n\n\n\n
for h in \/dev\/hidraw*; do echo -n \"$h: \"; udevadm info -q property -n $h | grep -E '^ID_(VENDOR|MODEL)_ID' | tr '\\n' ' '; echo; done<\/code><\/pre>\n\n\n\n\/dev\/hidraw0: ID_MODEL_ID=0001 ID_VENDOR_ID=0627\n\/dev\/hidraw1: ID_MODEL_ID=0012 ID_VENDOR_ID=0d8c<\/code><\/pre>\n\n\n\nHere \/dev\/hidraw1<\/code> is the interface. The number can change between boots, so the udev rule in the PTT arbiter section creates a stable name. Interfaces that key through a serial port or by VOX have no HID node, and the section on other PTT interfaces covers them.<\/p>\n\n\n\nService Accounts<\/h2>\n\n\n\n Each program runs under its own unprivileged account. The accounts share the audio<\/code> group so that all of them can open the same sound devices. The Dire Wolf package already creates a direwolf<\/code> account.<\/p>\n\n\n\nsudo usermod -aG audio direwolf\nsudo useradd --system --no-create-home --shell \/usr\/sbin\/nologin --user-group pttmux\nsudo usermod -aG audio,plugdev pttmux\nsudo useradd --system --home-dir \/opt\/linbpq --no-create-home --shell \/usr\/sbin\/nologin --user-group linbpq<\/code><\/pre>\n\n\n\nThe xara<\/code> account is created by the XARA installer, and the administrator account joins the audio<\/code> group so that it can run the audio tests.<\/p>\n\n\n\nsudo usermod -aG audio USERNAME<\/code><\/pre>\n\n\n\nThe group change applies to new login sessions, so a fresh SSH login follows this step.<\/p>\n\n\n\n
Sharing the Radio Audio Between Modems (ALSA dsnoop and dmix)<\/h2>\n\n\n\n A USB sound card opened directly by one program is unavailable to the next program. Three modems on one radio need the opposite: every modem must hear the same received audio, and every modem must be able to transmit through the same output. ALSA provides both with two plugins.<\/p>\n\n\n\n
\nThe dsnoop plugin takes the single capture stream from the interface and hands a copy to every program that opens it.<\/li>\n\n\n\n The dmix plugin accepts playback streams from several programs and mixes them into the single stream that goes to the interface.<\/li>\n<\/ul>\n\n\n\nPrograms open the shared devices by name instead of opening the hardware. The names are defined in \/etc\/asound.conf<\/code>.<\/p>\n\n\n\nsudo nano \/etc\/asound.conf<\/code><\/pre>\n\n\n\nThe file contains the following.<\/p>\n\n\n\n
# Shared access to the Allscan URI 125 (C-Media CM108) for several modems at once.\n#\n# capture : one hardware stream fanned out to every reader (dsnoop)\n# playback: every writer mixed into one hardware stream (dmix)\n#\n# ipc_perm\/ipc_gid let different service users (direwolf, xara, ...) share the\n# same dsnoop\/dmix instance; they must all be in group \"audio\".\n#\n# IMPORTANT: keep ipc_key values well apart (>= 10). dmix also uses key+1 for its\n# sum buffer and dsnoop's segment lands at a nearby key; adjacent keys (e.g. 1 and 2)\n# collide and dmix then fails with \"unable to initialize sum ring buffer\".\n#\n# RX\/TX audio levels: set with alsamixer\/amixer on card \"Device\" (AGC must be off).\n\npcm.uri_capture {\n type dsnoop\n ipc_key 584410\n ipc_perm 0660\n ipc_gid audio\n slave {\n pcm \"hw:CARD=Device,DEV=0\"\n channels 1\n rate 48000\n format S16_LE\n period_size 480\n buffer_size 4800\n }\n}\n\npcm.uri_playback {\n type dmix\n ipc_key 584420\n ipc_perm 0660\n ipc_gid audio\n slave {\n pcm \"hw:CARD=Device,DEV=0\"\n channels 2\n rate 48000\n format S16_LE\n period_size 480\n # 200 ms: Xara insists the playback device buffer >= 4x its tx-buffer-ms (40 ms\n # default -> 160 ms; its minimum setting of 30 ms still needs 120 ms).\n buffer_size 9600\n }\n bindings {\n 0 0\n 1 1\n }\n}\n\n# --- Dire Wolf: stereo pair = two modems (left=1200 baud, right=9600 baud) -----\n# RX: the single mono radio audio is copied to both channels.\npcm.dw_in {\n type plug\n slave.pcm {\n type route\n slave.pcm \"uri_capture\"\n slave.channels 1\n ttable.0.0 1\n ttable.1.0 1\n }\n}\n# TX: left and right are summed onto both radio channels.\npcm.dw_out {\n type plug\n slave.pcm {\n type route\n slave.pcm \"uri_playback\"\n slave.channels 2\n ttable.0.0 1\n ttable.0.1 1\n ttable.1.0 1\n ttable.1.1 1\n }\n}\n\n# --- Xara (mono modem) ---------------------------------------------------------\npcm.xara_in {\n type plug\n slave.pcm \"uri_capture\"\n}\npcm.xara_out {\n type plug\n slave.pcm {\n type route\n slave.pcm \"uri_playback\"\n slave.channels 2\n ttable.0.0 1\n ttable.0.1 1\n }\n}<\/code><\/pre>\n\n\n\nThe file defines six names, and each one has a specific job.<\/p>\n\n\n\n
\nuri_capture<\/code> is the dsnoop device. It reads the interface once, as mono at 48000 Hz.<\/li>\n\n\n\nuri_playback<\/code> is the dmix device. It writes to the interface as stereo at 48000 Hz.<\/li>\n\n\n\ndw_in<\/code> gives Dire Wolf a stereo capture stream. The mono radio audio is copied to both channels, so the 1200 baud modem on the left channel and the 9600 baud modem on the right channel both receive everything the radio hears.<\/li>\n\n\n\ndw_out<\/code> accepts Dire Wolf’s stereo output. The left channel (1200 baud modem) and the right channel (9600 baud modem) are both summed onto both channels of the interface, so either modem reaches the radio.<\/li>\n\n\n\nxara_in<\/code> and xara_out<\/code> give XARA a mono capture and playback path on the same shared devices.<\/li>\n<\/ul>\n\n\n\nTwo numbers in the file matter for the rest of the build.<\/p>\n\n\n\n
The ipc_key<\/code> values are 584410 and 584420. ALSA keeps shared state in System V shared memory, and dmix uses the key plus one for its mixing buffer, while dsnoop’s segment lands at a nearby key. Keys at least ten apart keep the two plugins from colliding.<\/p>\n\n\n\nThe dmix buffer_size<\/code> is 9600 frames, which is 200 ms at 48000 Hz. XARA requires the playback device to buffer at least four times its transmit buffer setting (40 ms by default, so 160 ms). The same buffer also determines the transmit tail time set later in the Dire Wolf and LinBPQ configuration.<\/p>\n\n\n\nThe ipc_perm 0660<\/code> and ipc_gid audio<\/code> lines let accounts in the audio<\/code> group share the same dsnoop and dmix instances.<\/p>\n\n\n\nALSA reads the file whenever a program opens a device. The new names appear in the device list.<\/p>\n\n\n\n
aplay -L | grep -E '^(uri_|dw_|xara_)'<\/code><\/pre>\n\n\n\nuri_capture\nuri_playback\ndw_in\ndw_out\nxara_in\nxara_out<\/code><\/pre>\n\n\n\nVerifying the shared devices<\/h3>\n\n\n\n The following script runs two captures and two playbacks at the same time under two different accounts. The playback is digital silence and PTT stays unkeyed, so nothing is transmitted.<\/p>\n\n\n\n
nano ~\/audiotest.py<\/code><\/pre>\n\n\n\n#!\/usr\/bin\/env python3\n\"\"\"Shared-audio check for the virtual ALSA devices.\n\nTwo different users capture the same radio audio at once and play digital\nsilence into the shared mixer at once. PTT is never keyed, so nothing is\ntransmitted. Run with: python3 audiotest.py\n\"\"\"\nimport array\nimport getpass\nimport math\nimport subprocess\nimport sys\nimport threading\n\nresults = {}\n\n\ndef rms(samples):\n return math.sqrt(sum(v * v for v in samples) \/ max(len(samples), 1))\n\n\ndef capture(user, device, channels):\n p = subprocess.run(\n [\"sudo\", \"-u\", user, \"arecord\", \"-q\", \"-D\", device, \"-f\", \"S16_LE\", \"-r\", \"48000\",\n \"-c\", str(channels), \"-d\", \"4\", \"-t\", \"raw\"], capture_output=True)\n data = array.array(\"h\")\n data.frombytes(p.stdout[: len(p.stdout) \/\/ 2 * 2])\n left = data[0::channels]\n right = data[1::channels] if channels > 1 else left\n results[f\"RX {user:10s} {device:12s}\"] = (\n p.returncode, f\"frames={len(data) \/\/ channels} rmsL={rms(left):.1f} rmsR={rms(right):.1f}\"\n + (f\" {p.stderr.decode().strip()}\" if p.stderr else \"\"))\n\n\ndef play(user, device, channels):\n p = subprocess.run(\n [\"sudo\", \"-u\", user, \"aplay\", \"-q\", \"-D\", device, \"-f\", \"S16_LE\", \"-r\", \"48000\",\n \"-c\", str(channels), \"-d\", \"4\", \"\/dev\/zero\"], capture_output=True)\n results[f\"TX {user:10s} {device:12s}\"] = (\n p.returncode, \"played 4 s of silence\" + (f\" {p.stderr.decode().strip()}\" if p.stderr else \"\"))\n\n\nme = getpass.getuser()\njobs = [\n (capture, (me, \"uri_capture\", 1)),\n (capture, (\"direwolf\", \"dw_in\", 2)),\n (play, (me, \"uri_playback\", 2)),\n (play, (\"direwolf\", \"dw_out\", 2)),\n]\nthreads = [threading.Thread(target=fn, args=args) for fn, args in jobs]\nfor t in threads:\n t.start()\nfor t in threads:\n t.join()\n\nfailed = 0\nfor key in sorted(results):\n code, message = results[key]\n print((\"OK \" if code == 0 else \"FAIL \") + key + \" \" + message)\n failed += code != 0\nsys.exit(1 if failed else 0)<\/code><\/pre>\n\n\n\npython3 -I ~\/audiotest.py<\/code><\/pre>\n\n\n\nOK RX direwolf dw_in frames=192000 rmsL=3444.5 rmsR=3444.5\nOK RX admin uri_capture frames=192000 rmsL=3444.4 rmsR=3444.4\nOK TX direwolf dw_out played 4 s of silence\nOK TX admin uri_playback played 4 s of silence<\/code><\/pre>\n\n\n\nFour OK<\/code> lines confirm that two accounts read and write the interface simultaneously. Identical rmsL<\/code> and rmsR<\/code> values on the dw_in<\/code> line confirm that the mono radio audio reaches both modem channels. A non-zero RMS value shows that the radio data port is delivering audio to the interface.<\/p>\n\n\n\nMixer Levels<\/h2>\n\n\n\n The interface exposes three mixer controls.<\/p>\n\n\n\n
amixer -c 0 scontrols<\/code><\/pre>\n\n\n\nSimple mixer control 'Speaker',0\nSimple mixer control 'Mic',0\nSimple mixer control 'Auto Gain Control',0<\/code><\/pre>\n\n\n\nData modems need a fixed, predictable audio path, so automatic gain control is switched off. The Speaker control sets the transmit audio level into the radio. The reference station settled on step 21 of 37 (-16 dB), which is a good starting point for the transmit level adjustment later in this article.<\/p>\n\n\n\n
amixer -c 0 sset 'Auto Gain Control' off\namixer -c 0 sset Speaker 21\nsudo alsactl store<\/code><\/pre>\n\n\n\namixer -c 0 sget Speaker | grep -E 'Limits|Front'<\/code><\/pre>\n\n\n\n Limits: Playback 0 - 37\n Front Left: Playback 21 [57%] [-16.00dB] [on]\n Front Right: Playback 21 [57%] [-16.00dB] [on]<\/code><\/pre>\n\n\n\nThe alsactl store<\/code> command saves the state, and the ALSA restore service reapplies it at every boot.<\/p>\n\n\n\nPTT Arbiter<\/h2>\n\n\n\nThe problem<\/h3>\n\n\n\n Dire Wolf keys PTT for the 1200 baud and 9600 baud channels independently, and XARA keys PTT for VARA FM. All three drive the one GPIO line of the interface. With all three modems driving the line directly, whichever modem finishes first releases PTT while another modem is still transmitting, and the end of that transmission is lost.<\/p>\n\n\n\n
The solution<\/h3>\n\n\n\n Each modem keys a named pipe (FIFO) instead of the real interface. A small program, the PTT arbiter, reads all the pipes and drives the interface itself. PTT is asserted while any modem wants to transmit and is released only after every modem has released.<\/p>\n\n\n\n
\nDire Wolf accepts any file path in its PTT CM108<\/code> setting, and XARA accepts any path in ptt = cm108:PATH:GPIO<\/code>. Both write the same five-byte CM108 HID report to that path: [0, 0, data, mask, 0]<\/code>, where the data bit for the GPIO is 1 for transmit and 0 for receive.<\/li>\n\n\n\nThe output side of the arbiter is selectable. The default writes the CM108 HID report to the real interface, and --output rts<\/code> or --output dtr<\/code> drives a serial port line instead, as covered in the section on other PTT interfaces.<\/li>\n\n\n\nThe arbiter keeps one state per modem, ignores reports for other GPIO pins, and logs the moments when two modems key at the same time as OVERLAP<\/code>.<\/li>\n\n\n\nPTT is forced off when the arbiter starts and when it exits. A per-modem watchdog releases a modem that holds PTT for more than 120 seconds.<\/li>\n\n\n\n The FIFOs survive an arbiter restart, because XARA holds its pipe open for as long as it runs. Stale bytes left in a pipe are discarded at start-up so that only fresh transmit requests key the radio.<\/li>\n\n\n\n The --dry-run<\/code> option logs the actions and leaves the interface untouched.<\/li>\n<\/ul>\n\n\n\nThe udev rule<\/h3>\n\n\n\n The HID node number can change between boots. A udev rule creates a stable name, \/dev\/uri125-ptt<\/code>, from the vendor and product IDs found earlier with lsusb<\/code>.<\/p>\n\n\n\nsudo nano \/etc\/udev\/rules.d\/61-uri125.rules<\/code><\/pre>\n\n\n\n# Stable name for the Allscan URI 125 (CM108) PTT HID node, independent of\n# \/dev\/hidrawN numbering. Used by pttmux.\nSUBSYSTEM==\"hidraw\", ATTRS{idVendor}==\"0d8c\", ATTRS{idProduct}==\"0012\", SYMLINK+=\"uri125-ptt\"<\/code><\/pre>\n\n\n\nsudo udevadm control --reload\nsudo udevadm trigger --subsystem-match=hidraw --action=change\nls -l \/dev\/uri125-ptt<\/code><\/pre>\n\n\n\nlrwxrwxrwx 1 root root 7 Nov 4 13:35 \/dev\/uri125-ptt -> hidraw1<\/code><\/pre>\n\n\n\nThe Dire Wolf package installs a second udev rule that gives the audio<\/code> group read and write access to CM108 HID nodes. The pttmux<\/code> account is a member of that group.<\/p>\n\n\n\nls -l \/dev\/hidraw1<\/code><\/pre>\n\n\n\ncrw-rw---- 1 root audio 240, 1 Nov 4 13:35 \/dev\/hidraw1<\/code><\/pre>\n\n\n\nThe arbiter program<\/h3>\n\n\n\n The script uses only the Python standard library. It is saved as \/usr\/local\/bin\/pttmux<\/code> and made executable.<\/p>\n\n\n\nsudo nano \/usr\/local\/bin\/pttmux\nsudo chmod 755 \/usr\/local\/bin\/pttmux<\/code><\/pre>\n\n\n\n#!\/usr\/bin\/env python3\n\"\"\"pttmux - merge PTT requests from several modems into one PTT line.\n\nSeveral modems (Dire Wolf 1200\/9600, XARA) share a single radio. Each modem is\npointed at its own FIFO, which looks like a \"CM108 hidraw device\" to it. A modem\nkeys the radio by writing the usual 5-byte CM108 HID output report\n\n [0, 0, data, mask, 0] data bit (gpio-1) = 1 -> PTT on\n\nto its FIFO. This daemon tracks one state per client and drives the real PTT\nline so that PTT is asserted while ANY client wants to transmit and released\nonly when ALL have released. Without this, whichever modem finishes first would\ndrop PTT in the middle of another modem's transmission.\n\nThe real PTT line is chosen with --output:\n\n cm108 GPIO of a CM108\/CM119 interface, through --device \/dev\/hidrawN (default)\n rts RTS of the serial port --device; DTR is held low\n dtr DTR of the serial port --device; RTS is held low\n\n--invert reverses the polarity of a serial line (keyed = line low).\n\nSafety: PTT is forced off at start-up and shutdown, and a per-client watchdog\nreleases a client that has held PTT longer than --max-on seconds.\n\nOverlapping requests (two modems transmitting at once) are logged; audio from the\nmodems is mixed by ALSA dmix, so an overlap is a collision on the air, exactly as\non any shared packet channel.\n\"\"\"\nimport argparse\nimport fcntl\nimport grp\nimport os\nimport select\nimport signal\nimport stat\nimport struct\nimport sys\nimport termios\nimport time\n\n\ndef log(msg):\n print(msg, flush=True) # journald adds the timestamp\n\n\nclass Cm108Output:\n \"\"\"CM108\/CM119 GPIO, driven with a HID output report.\"\"\"\n\n def __init__(self, device, bit):\n self.device, self.bit = device, bit\n self.fd = os.open(device, os.O_WRONLY)\n\n def describe(self):\n return f\"CM108 GPIO bit {self.bit:#04x} on {self.device}\"\n\n def set(self, on):\n os.write(self.fd, bytes([0, 0, self.bit if on else 0, self.bit, 0]))\n\n def close(self):\n os.close(self.fd)\n\n\nclass SerialOutput:\n \"\"\"RTS or DTR of a serial port; the other line is held low.\"\"\"\n\n def __init__(self, device, line, invert):\n self.device, self.line, self.invert = device, line, invert\n self.fd = os.open(device, os.O_RDWR | os.O_NOCTTY | os.O_NONBLOCK)\n try:\n if not os.isatty(self.fd):\n raise OSError(f\"{device} is not a serial port\")\n # Opening a port raises RTS and DTR; both are cleared at once.\n self._modem(termios.TIOCMBIC, termios.TIOCM_RTS | termios.TIOCM_DTR)\n # Keep the lines as they are when the port closes (the default would\n # drop them, which keys an inverted interface).\n try:\n attrs = termios.tcgetattr(self.fd)\n attrs[2] &= ~termios.HUPCL\n termios.tcsetattr(self.fd, termios.TCSANOW, attrs)\n except termios.error:\n if invert:\n raise\n self.set(False)\n except BaseException:\n os.close(self.fd)\n raise\n\n def describe(self):\n name = \"RTS\" if self.line == termios.TIOCM_RTS else \"DTR\"\n pol = \"inverted\" if self.invert else \"normal\"\n return f\"{name} ({pol} polarity, other line held low) on {self.device}\"\n\n def _modem(self, request, bits):\n fcntl.ioctl(self.fd, request, struct.pack(\"I\", bits))\n\n def _lines(self):\n return struct.unpack(\"I\", fcntl.ioctl(self.fd, termios.TIOCMGET, struct.pack(\"I\", 0)))[0]\n\n def set(self, on):\n high = on != self.invert\n self._modem(termios.TIOCMBIS if high else termios.TIOCMBIC, self.line)\n if bool(self._lines() & self.line) != high:\n raise OSError(f\"{self.device}: modem line did not follow\")\n\n def close(self):\n os.close(self.fd)\n\n\nclass Mux:\n def __init__(self, a):\n self.args = a\n self.bit = 1 << (a.gpio - 1)\n self.dry = a.dry_run\n self.max_on = a.max_on\n self.stop = False\n self.out = None\n self.hw_on = None # unknown until first write\n self.last_err = (\"\", 0.0)\n self.fd = {} # client -> fifo fd\n self.name = {} # fifo fd -> client\n self.on_since = {} # client -> monotonic time PTT went on, or None\n self.buf = {}\n gid = grp.getgrnam(a.group).gr_gid if a.group else -1\n os.makedirs(a.dir, exist_ok=True)\n for c in a.client:\n p = os.path.join(a.dir, c)\n # Reuse an existing FIFO: a modem (xara) keeps its end open for its whole\n # life, so replacing the inode on a pttmux restart would orphan it.\n try:\n if not stat.S_ISFIFO(os.stat(p).st_mode):\n os.unlink(p)\n raise FileNotFoundError\n except FileNotFoundError:\n os.mkfifo(p, 0o660)\n os.chmod(p, 0o660)\n if gid != -1:\n os.chown(p, -1, gid)\n # O_RDWR: we are always a reader (so a modem's open() never blocks)\n # and a writer (so we never see EOF when modems come and go).\n fd = os.open(p, os.O_RDWR | os.O_NONBLOCK)\n self.fd[c] = fd\n self.name[fd] = c\n self.on_since[c] = None\n self.buf[c] = b\"\"\n log(f\"client {c}: {p}\")\n\n # ---- real PTT line ----------------------------------------------------\n def open_output(self):\n a = self.args\n if a.output == \"cm108\":\n out = Cm108Output(a.device, self.bit)\n else:\n line = termios.TIOCM_RTS if a.output == \"rts\" else termios.TIOCM_DTR\n out = SerialOutput(a.device, line, a.invert)\n log(\"output: \" + out.describe())\n return out\n\n def drop_output(self):\n if self.out is not None:\n try:\n self.out.close()\n except OSError:\n pass\n self.out = None\n\n def hw(self, on):\n if self.dry:\n self.hw_on = on\n log(f\"[dry-run] PTT line would be {'ON' if on else 'OFF'}\")\n return\n try:\n if self.out is None:\n self.out = self.open_output()\n self.out.set(on)\n self.hw_on = on\n except OSError as e:\n msg, when = self.last_err\n now = time.monotonic()\n if str(e) != msg or now - when >= 30: # one line per 30 s, not one per retry\n log(f\"ERROR on {self.args.device}: {e}\")\n self.last_err = (str(e), now)\n self.drop_output()\n\n def want(self):\n return any(t is not None for t in self.on_since.values())\n\n def reconcile(self):\n w = self.want()\n if w != self.hw_on:\n self.hw(w)\n if self.hw_on == w:\n log(\"PTT \" + (\"ON\" if w else \"OFF\"))\n\n # ---- client reports ---------------------------------------------------\n def report(self, c, rep):\n data, mask = rep[2], rep[3]\n if not mask & self.bit:\n return # not about our GPIO\n on = bool(data & self.bit)\n was = self.on_since[c] is not None\n if on and not was:\n others = [o for o, t in self.on_since.items() if t is not None and o != c]\n self.on_since[c] = time.monotonic()\n log(f\"{c} keyed\" + (f\" ** OVERLAP with {','.join(others)} **\" if others else \"\"))\n elif not on and was:\n held = time.monotonic() - self.on_since[c]\n self.on_since[c] = None\n log(f\"{c} released after {held:.1f}s\")\n\n def feed(self, c, data):\n b = self.buf[c] + data\n while len(b) >= 5:\n rep = b[:5]\n if rep[0] != 0 or rep[4] != 0: # not a CM108 report: resync\n b = b[1:]\n continue\n b = b[5:]\n self.report(c, rep)\n self.buf[c] = b\n\n def watchdog(self):\n now = time.monotonic()\n for c, t in self.on_since.items():\n if t is not None and now - t > self.max_on:\n log(f\"WATCHDOG: {c} held PTT > {self.max_on}s, forcing release\")\n self.on_since[c] = None\n\n # ---- main loop --------------------------------------------------------\n def run(self):\n p = select.poll()\n for fd in self.name:\n p.register(fd, select.POLLIN)\n self.hw(False) # known-safe start\n # A reused FIFO may hold stale reports (e.g. an old PTT-ON written just before we\n # were restarted). Never replay those: discard everything, fail safe = PTT off.\n for fd, c in self.name.items():\n drained = 0\n while True:\n try:\n d = os.read(fd, 4096)\n except BlockingIOError:\n break\n if not d:\n break\n drained += len(d)\n if drained:\n log(f\"discarded {drained} stale byte(s) from {c}\")\n log(\"ready; PTT off\")\n while not self.stop:\n try:\n events = p.poll(500)\n except InterruptedError:\n continue\n for fd, _ in events:\n try:\n data = os.read(fd, 4096)\n except BlockingIOError:\n continue\n if data:\n self.feed(self.name[fd], data)\n self.watchdog()\n self.reconcile()\n\n def shutdown(self):\n for c in self.on_since:\n self.on_since[c] = None\n self.hw(False)\n self.drop_output()\n log(\"PTT forced OFF; exiting\")\n\n\ndef main():\n ap = argparse.ArgumentParser(description=__doc__.split(\"\\n\\n\")[0])\n ap.add_argument(\"--output\", choices=(\"cm108\", \"rts\", \"dtr\"), default=\"cm108\",\n help=\"real PTT line: CM108 GPIO (default) or serial RTS\/DTR\")\n ap.add_argument(\"--device\", default=\"\/dev\/uri125-ptt\",\n help=\"hidraw node (cm108) or serial port (rts, dtr)\")\n ap.add_argument(\"--gpio\", type=int, default=3, choices=range(1, 9), metavar=\"1-8\",\n help=\"GPIO number the modems key in their CM108 reports (default 3)\")\n ap.add_argument(\"--invert\", action=\"store_true\", help=\"serial output: keyed = line low\")\n ap.add_argument(\"--dir\", default=\"\/run\/pttmux\", help=\"directory for client FIFOs\")\n ap.add_argument(\"--group\", default=\"audio\", help=\"group given access to the FIFOs\")\n ap.add_argument(\"--client\", action=\"append\", required=True, help=\"client name (repeatable)\")\n ap.add_argument(\"--max-on\", type=float, default=120.0, help=\"per-client watchdog, seconds\")\n ap.add_argument(\"--dry-run\", action=\"store_true\", help=\"never touch the PTT line; log only\")\n a = ap.parse_args()\n if a.invert and a.output == \"cm108\":\n ap.error(\"--invert applies to the serial outputs (rts, dtr)\")\n\n m = Mux(a)\n\n def bye(*_):\n m.stop = True\n\n signal.signal(signal.SIGTERM, bye)\n signal.signal(signal.SIGINT, bye)\n try:\n m.run()\n finally:\n m.shutdown()\n\n\nif __name__ == \"__main__\":\n sys.exit(main())<\/code><\/pre>\n\n\n\nThe service<\/h3>\n\n\n\n The service runs the arbiter as the pttmux<\/code> account with the three FIFO names dw1200<\/code>, dw9600<\/code>, and xara<\/code>. The RuntimeDirectory<\/code> setting creates \/run\/pttmux<\/code> at start. The default --output cm108<\/code> writes to the HID node named by --device<\/code>. The --gpio 3<\/code> option names the GPIO number that the modems key in their pipes and that the CM108 output drives, which is 3 on the URI 125 and on most CM108 interfaces.<\/p>\n\n\n\nsudo nano \/etc\/systemd\/system\/pttmux.service<\/code><\/pre>\n\n\n\n[Unit]\nDescription=PTT multiplexer (shared by Dire Wolf and XARA)\nAfter=sound.target\n\n[Service]\nType=simple\nUser=pttmux\nGroup=pttmux\nSupplementaryGroups=audio plugdev\nRuntimeDirectory=pttmux\nRuntimeDirectoryMode=0755\nRuntimeDirectoryPreserve=restart\nExecStart=\/usr\/local\/bin\/pttmux --device \/dev\/uri125-ptt --gpio 3 --dir \/run\/pttmux --client dw1200 --client dw9600 --client xara\nRestart=always\nRestartSec=2\n\nNoNewPrivileges=yes\nProtectSystem=strict\nProtectHome=yes\nPrivateTmp=yes\nProtectKernelTunables=yes\nProtectKernelModules=yes\nProtectControlGroups=yes\nRestrictNamespaces=yes\nRestrictSUIDSGID=yes\nLockPersonality=yes\nRestrictAddressFamilies=AF_UNIX\nCapabilityBoundingSet=\nDevicePolicy=closed\nDeviceAllow=char-hidraw rw\nUMask=0007\n\n[Install]\nWantedBy=multi-user.target<\/code><\/pre>\n\n\n\nsudo systemctl daemon-reload\nsudo systemctl enable --now pttmux<\/code><\/pre>\n\n\n\nCreated symlink '\/etc\/systemd\/system\/multi-user.target.wants\/pttmux.service' \u2192 '\/etc\/systemd\/system\/pttmux.service'.<\/code><\/pre>\n\n\n\nThe service state, the three pipes, and the start-up log confirm the arbiter.<\/p>\n\n\n\n
systemctl is-active pttmux\nls -l \/run\/pttmux\/\nsudo journalctl -u pttmux -n 6 --no-pager -o cat<\/code><\/pre>\n\n\n\nactive\ntotal 0\nprw-rw---- 1 pttmux audio 0 Nov 4 13:35 dw1200\nprw-rw---- 1 pttmux audio 0 Nov 4 13:35 dw9600\nprw-rw---- 1 pttmux audio 0 Nov 4 13:35 xara\nStarted pttmux.service - PTT multiplexer (shared by Dire Wolf and XARA).\nclient dw1200: \/run\/pttmux\/dw1200\nclient dw9600: \/run\/pttmux\/dw9600\nclient xara: \/run\/pttmux\/xara\noutput: CM108 GPIO bit 0x04 on \/dev\/uri125-ptt\nready; PTT off<\/code><\/pre>\n\n\n\nKeying the radio by hand<\/h3>\n\n\n\n A short manual key-down confirms that GPIO 3 is the right pin for the interface and that the radio transmits. The test is run into a dummy load. Writing the report bytes to a pipe acts exactly like a modem keying PTT.<\/p>\n\n\n\n
sudo sh -c 'printf \"\\000\\000\\004\\004\\000\" > \/run\/pttmux\/xara'\nsleep 2\nsudo sh -c 'printf \"\\000\\000\\000\\004\\000\" > \/run\/pttmux\/xara'<\/code><\/pre>\n\n\n\nThe first command sends data 04<\/code> with mask 04<\/code> (GPIO 3 on), and the second sends data 00<\/code> with the same mask (GPIO 3 off). The radio keys for two seconds, and the arbiter logs the event.<\/p>\n\n\n\nsudo journalctl -u pttmux -n 4 --no-pager -o cat<\/code><\/pre>\n\n\n\nxara keyed\nPTT ON\nxara released after 2.0s\nPTT OFF<\/code><\/pre>\n\n\n\nAn interface that keys on a different GPIO number takes that number in the --gpio<\/code> option of the service and in the Dire Wolf and XARA PTT settings that follow. For GPIO number n, the data and mask bytes of the manual test are both 2 raised to the power n minus 1 (4 for GPIO 3, 8 for GPIO 4).<\/p>\n\n\n\nOther PTT Interfaces: Serial Port and VOX<\/h2>\n\n\n\n The CM108 GPIO is one of three common ways for a sound interface to key a radio. Interfaces such as the DigiRig key the radio with a control line of a USB serial port. Interfaces such as the Tigertronics SignaLink USB key the radio from the audio itself (VOX). Each method fits the same station design with a small change.<\/p>\n\n\n\nInterface type<\/th> Example<\/th> How the radio is keyed<\/th> PTT arbiter<\/th> Dire Wolf PTT line<\/th> XARA ptt setting<\/th><\/tr><\/thead> CM108 GPIO<\/td> Allscan URI 125<\/td> GPIO pin of the sound chip<\/td> --output cm108<\/code><\/td>PTT CM108 3 \/run\/pttmux\/NAME<\/code><\/td>cm108:\/run\/pttmux\/xara:3<\/code><\/td><\/tr>Serial port<\/td> DigiRig<\/td> RTS (or DTR) of the interface’s serial port<\/td> --output rts<\/code> (or dtr<\/code>)<\/td>the same pipe line as CM108<\/td> the same pipe setting as CM108<\/td><\/tr> VOX<\/td> SignaLink USB<\/td> audio from the computer trips a voice-operated circuit<\/td> not used<\/td> no PTT line<\/td> vox<\/code><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nSerial Port PTT (DigiRig and Similar)<\/h3>\n\n\n\n The DigiRig Mobile pairs a CM108-class sound codec with a CP2102 USB serial port. The product page describes its PTT as an open-collector switch controlled by the RTS signal of the COM port. The radio is keyed by raising RTS, and the sound card carries only audio.<\/p>\n\n\n\n
The modems keep keying their pipes exactly as in the CM108 build, and the Dire Wolf and XARA PTT settings stay unchanged. The arbiter performs the last step: it turns the combined PTT state into RTS (or DTR) on the serial port, using the same line sequence as XARA’s own serial PTT. RTS or DTR is the keyed line, the other line is held low, and both lines are cleared within milliseconds of the port opening, because Linux raises them at open. When the arbiter exits, the lines stay in their released state.<\/p>\n\n\n\n
The serial port appears under \/dev\/serial\/by-id\/<\/code> with a name that stays the same between boots and across USB ports.<\/p>\n\n\n\nls -l \/dev\/serial\/by-id\/<\/code><\/pre>\n\n\n\nlrwxrwxrwx 1 root root 13 Nov 4 13:35 usb-Silicon_Labs_CP2102N_USB_to_UART_Bridge_Controller_SERIAL-if00-port0 -> ..\/..\/ttyUSB0<\/code><\/pre>\n\n\n\nThe example shows the naming of a CP210x device. The serial number part of the name differs for every device. The full path, \/dev\/serial\/by-id\/usb-Silicon_Labs_CP2102N_USB_to_UART_Bridge_Controller_SERIAL-if00-port0<\/code> in this example, is the SERIAL_PORT<\/code> value used below.<\/p>\n\n\n\nSerial devices belong to the dialout<\/code> group, so the arbiter account joins that group.<\/p>\n\n\n\nsudo usermod -aG dialout pttmux<\/code><\/pre>\n\n\n\nThe sound card of the interface is named in \/etc\/asound.conf<\/code>. A CM108-class codec captures in mono, so the file from the earlier section applies with the card name changed. The section on sound card changes at the end of this section covers the card name and the channel count.<\/p>\n\n\n\nThree settings in \/etc\/systemd\/system\/pttmux.service<\/code> change. SupplementaryGroups<\/code> adds the dialout<\/code> group, ExecStart<\/code> selects the serial output and names the port, and DeviceAllow<\/code> permits the serial device classes in place of the HID class.<\/p>\n\n\n\nSupplementaryGroups=audio dialout\nExecStart=\/usr\/local\/bin\/pttmux --output rts --device \/dev\/serial\/by-id\/SERIAL_PORT --dir \/run\/pttmux --client dw1200 --client dw9600 --client xara\nDeviceAllow=char-ttyUSB rw\nDeviceAllow=char-ttyACM rw<\/code><\/pre>\n\n\n\nThe original SupplementaryGroups=audio plugdev<\/code>, ExecStart<\/code>, and DeviceAllow=char-hidraw rw<\/code> lines are replaced by the lines above. The udev rule for the CM108 HID node is unnecessary for a serial interface.<\/p>\n\n\n\nThe options of the serial output are:<\/p>\n\n\n\n
\n--output rts<\/code> keys with RTS and holds DTR low. --output dtr<\/code> keys with DTR and holds RTS low.<\/li>\n\n\n\n--invert<\/code> reverses the polarity for interfaces that key when the line is low. The idle state is then a high line, and the arbiter keeps it high when it exits.<\/li>\n\n\n\n--device<\/code> names the serial port. --gpio<\/code> keeps its default of 3, because it names the GPIO number that the modems key in their pipes.<\/li>\n<\/ul>\n\n\n\n